This could be an issue with the famous K1 accumulator spring (search the forum for it). It'll cause extremely crappy 2-3 engagement with late engagement, really sloppy/slow engagement, or flaring during the shift.

My SDL had hideously horrible shifting before I serviced the transmission. K1 accumulator spring was broken into 3 pieces. Changed that out, new filter and cheap-o Valvoline Dex III from Wal-Fart and it drives like a completely different car, nice crisp and firm gear changes. No more sloppy engagement or horribly late/hung shifts. 3-4 still happens a bit early for my liking in slow traffic, but I attribute that to a gearbox with 200K+ on it.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]